The organic geochemistry of hydrocarbon-rich core material recovered by the CSDP is examined to establish whether hydrocarbons are associated with the migration and emplacement of organic matter by post-impact hydrothermal activity

Gilmour, Iain

Morgan, J. V.

Sephton, M. A.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsOrganic Geochemistry of a Hydrocarbon-richCalcarenite from the Chicxulub Scientific DrillingProgramConference or Workshop ItemHow to cite:Gilmour, Iain; Sephton, M. A. and Morgan, J. V. (2003). Organic Geochemistry of a Hydrocarbon-rich Calcarenitefrom the Chicxulub Scientific Drilling Program. In: 34th Lunar and Planetary Science Conference, 17-21 Mar 2003,Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1771.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukORGANIC GEOCHEMISTRY OF A HYDROCARBON-RICH CALCARENITE FROM THECHICXULUB SCIENTIFIC DRILLING PORGRAM I. Gilmour1, M.A. Sephton and J.V. Morgan2, 1 Planetaryand Space Sciences Research Institute, The Open University, Milton Keynes MK7 6AA, U.K.(I.Gilmour@open.ac.uk), 2 Department of Earth Science and Engineering, Imperial College of Science Technologyand Medicine, Prince Consort Road, London SW7 2BP, United KingdomIntroduction: Terrestrial impact craters appear tobe unique in the geological environment in that theycontain evidence for a wide variety of carbonaceousphases ranging from shock-produced diamonds [1] tohydrocarbons in fluid inclusions [2]. Some of thesematerials, such as diamond, are a direct result of theimpact process. However, at some craters, carbona-ceous material has been incorporated into impactitesand other rocks post-impact, possibly as a result ofhydrothermal activity [3]. Part of the rationale for theChicxulub Scientific Drilling Project (CSDP) was toinvestigate the post-impact environment associatedwith such a large structure, seismic data indicates thediameter of the transient cavity was around 100 km[4]. The instigation of hydrothermal systems followingan impact and the consequent mobilization and migra-tion of organic matter and other biologically importantnutrients is one mechanism that has been proposed forthe development of potential habitats for primitive or-ganisms on Mars [5].At around a depth of 1.4 km the CSDP recoveredcore material that was rich in hydrocarbons. This sec-tion of the core consists of a laminated black cal-carenite that contains intraclasts, synsedimentary mi-crofaulting and soft sedimentary deformation features.This study examines the organic geochemistry of thishydrocarbon-rich material in an attempt to ascertainwhether these hydrocarbons are associated with themigration and emplacement of organic matter by post-impact hydrothermal activity. The sample used for ourinvestigation is number 1756 from core box 419 andcomes from a depth of 1405.085 m.Methods: Crushed and powdered samples of the hy-drocarbon-bearing calcarenite were solvent extractedusing dichloromethane (10 ml, Distol grade) by soni-cation for 30 minutes and separated into aliphatic andaromatic-rich fractions using column chromatography.The fractions were concentrated to around 50 ml undera stream of dry nitrogen and around 1 ml injected usedfor organic analysis. Analyses were by splitless injec-tion into a BPX5 capillary column (30 m x 0.25 mm x0.25 µm). The GC oven was operated under the fol-lowing conditions: initial isothermal hold for 2 min at50°C; temperature programmed at 5°C/min to 300°Cand final isothermal hold for 8 min.For pyrolysis around 30 mg of powdered samplewas loaded into a pre-cleaned quartz glass tube.  Thesample was flash pyrolysed at 610°C using a CDS1000 pyroprobe (CDS Analytical, Oxford, PA), within15 s after insertion into the interface, for 15 s in a flowof helium. The heating rate of pyrolysis was 20°C ms-1.The interface was held at 260°C and the GC injectormaintained at 250°C.Compound detection and identification was per-formed by gas chromatography-mass spectrometryusing an Agilent Technologies 5973 GCMS operatedin fullscan electron ionization mode (50-500 Da, 1.8scan s-1, 70 eV electron energy). GC column conditionswere the same as for the solvent extracted samples.Results: The total ion current chromatogram for a py-rolysate from the calcarenite sample is shown in Figure1. The trace is dominated by a large unresolved com-plex mixture (UCM), with the main resolved compo-nents being alkanes, aromatics (including significantamounts of naphthenic compounds) and sulfur con-taining thiophenes. Figure 2 shows a selected ion masschromatogram indicating the distribution of n-alkanesin the pyrolysate of the calcarenite sample. A distribu-tion of n-alkanes from n-C12 to n-C29 is observed to-gether with significant abundances of the isoprenoidspristane and phytane.10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.00Retention time (mins)ResponseUCMFigure 1 Total ion current chromatogram of pyrolysate fromsample 1756, core box 419, a calcarenite from a depth of1405.085 mFrom the total ion current chromatogram it was ap-parent that there were significant amounts of PAHspresent in the calcarenite. Figure 3 shows a recon-structed ion chromatogram of a portion of the TICwhere phenanthene/anthrancene and alkyl-substitutedphenanthrenes eluted.Lunar and Planetary Science XXXIV (2003) 1771.pdfANALYSIS OF HYDROCARBONS IN THE CHICXULUB IMPACT STRUCTURE:  I. Gilmour et al.Carbon in impact structures: Under ‘normal’ geo-logical conditions the formation and migration of hy-drocarbons is a slow process related to the subsidenceof sedimentary basins and the gradual maturation oforganic matter. However, in hydrothermal systems thematuration of organic matter, the generation of hydro-carbons, their expulsion and migration effectively takeplace in an instantaneous geological time frame.  Pre-vious studies of hydrothermally generated hydrocar-bons [see 6 for a review] suggest that they have a vari-ety of features that can distinguish them from ‘normal’petroleums For example: A broad distribution of n-alkanes with no carbon number predominance, anaphthenic hump, unresolved complex mixture, sig-nificant concentrations of pristane and phytane, maturebiomarker distributions and high concentrations ofPAHs and thio-PAHs. We observe some of these char-acteristics in the Chicxulub calcarenite: abundantPAHs and thio-PAHs, pristane, phytane and a broaddistribution of n-alkanes.Carbonaceous material has previously been foundassociated with several impact structures, most notablythe Sudbury impact structure in Canada and the Gard-nos impact structure in Norway. However, at boththese structures the carbon occurs as a severely de-hydrogenated graphitic or amorphous carbonaceousmaterial and its method of emplacement has provenproblematic to determine. Continued examination ofhydrocarbons associated with the Chicxulub structuremay indicate the feasibility of hydrothermal genera-tion, expulsion and migration of hydrocarbons as ameans of mobilizing, processing and concentratingorganic matter on early Earth and Mars.References: [1] Gilmour I. (1998) Spec. Pub. Geol. Soc.Lond. 140, 205-216. [2] Andersen T. and Burke E. A. J.(1996) Europ. J. Min.  8, 927-936.  [3] Gilmour I. et al.(2003) Geochim. Cosmochim. Acta In Press. [4] MorganJ.V., Warner M.R., Chicxulub Working Group.!!(1997) Na-ture, 390, 472-476. [5] Rathburn J.A. and Squyres S.W.(2202) Icarus, 157, 362-372. [6] Simoneit B.R.T (1993) In:Organic Geochemistry Principles and Applications (Eds.Engle M.A. and Macko S.A.)  397- 418.10 15 20 25 30 35 40 45 50Retention time (mins)Response222018161424PrPh1719151321232526 Figure 2 Mass chromatogram (m/z 99) showing the distribu-tion of n-alkanes in sample 1756, core box 419, a calcerinitefrom a depth of 1405.085 m. Pr = pristane, Ph = Phytane.25 27 29 31 33 35 37 39 41Retention time (mins)ResponseC1UCM Figure 2 Reconstructed ion chromatogram of the parentaland alkyl-phenanthrenes in sample 1756.Lunar and Planetary Science XXXIV (2003) 1771.pdf

Organic Geochemistry of a Hydrocarbon-rich Calcarenite from the Chicxulub Scientific Drilling Program

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Organic Geochemistry of a Hydrocarbon-rich Calcarenite from the Chicxulub Scientific Drilling Program

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